New transforming murine type C viruses were isolated and characterized with respect to target cell specificity in vivo, ability to transform cells in culture and their genome structure: (a) A lung carcinoma-associated virus (LCAV) was molecularly cloned and shown to have a mink cell focus-forming (MCF)-type structure. Transformed mink lung cells harboring this virus were sensitized to EGF suggesting that the mechanism of transformation by LCAV may involve sensitization of alveologenic lung cells to EGF. A virus-induced lung carcinoma cell line was characterized and shown to be derived from alveolar type I cells; (b) A new sarcoma virus was isolated and purified by the derivation of rescuable non-producer transformed mouse and rat cells. The translational products of this virus were P90 and P75 polyproteins which contained viral p15 and p12 structural proteins fused to a presumptive tumor gene product lacking tyrosine-specific phosphokinase activity. The genome of this virus, which does not contain the mammalian viral onc genes Ha-ras, v-abl, v-fes or v-mos, was molecularly cloned; (c) Several differentiating clonal lines were derived from a cell line established from a virus-induced ovarian carcinoma.